Variable-frequency relaxation oscillator



July 6, 1954 M. E. MOHR VARIABLE-FREQUENCY RELAXATION OSCILLATOR Original Filed June 2, 1949 4 Sheets-Sheet 2 ATTORNEY July 6, 1954 M. E. MOHR VARIABLE-FREQUENCY RELAXATION OSCILLATOR 4 Sheets-Sheet 3 Original Filed June 2, 1949 kbQRbO lNl/ENTOR By ME. MOHR 'QM Q ATTORNEY July 6, 1954 M. E. MOHR VARIABLE-FREQUENCY RELAXATION OSCILLATOR 4 Sheets-Sheet 4 Original Filed June 2, 1949 M M We a M. W Y Q B VUQ A TTQPA/E y Patented July 6, I954 VARIABLE-FREQUENCY RELAXATION OSCILLATOR Milton E. Mohr, Pacific Palisades, Calif., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application June 2, 1949, Serial No.

Divided and this application August 28, 1951, Serial No. 243,958

9 Claims.

This is a division of application Serial No. 96,706, filed June 2, 1949, now United States Patent No. 2,593,330 of April 15, 1952, for variable-frequency relaxation oscillator.

This invention relates to electrical systems and more particularly to oscillatory systems in which the frequency of oscillation may be changed between predetermined limits.

In the electric signaling art, it is frequently desirable to determine the frequency composition of a complex signal wave, such as a speech signal Wave. In such waves the instantaneous amplitude may rapidly vary in a complex manner dependent upon the instantaneous phase relations of a plurality of difierent wave components, each of which represents a different sound in the original signal; or a different frequency, or narrow band of frequencies, in the electrical equivalent of the original signal. The determined frequency composition may be utilized in a number of Ways, one of which is to depict graphically the varying composition of the wave in spectrographic form, such that the dimensional coordinates of the graph represented frequency and time, respectively, and the degree of brightness of each coordinate pcint in a visual graphic representation indicates the mean intensity of a particular frequency component at a particular instant of time. A system in which a complex signal wave may be dissected and graphically displayed in the abovedescribed manner is disclosed in United States Patent 2,403,986, dated July 16, 1946, to Lester Y. Lacy, reference to which is hereby made.

In systems of this type a complex signal wave is modulated with the output of an oscillator, the frequency of which is swept, or varied, between predetermined limits, and one of the sideband products of modulation is applied to the input of a relatively narrow band-pass filter. The frequency of the oscillator is varied such that the desired sideband is repeatedly moved, or swept, across the pass-band frequency of the filter. The effect of this action is that the filter virtually repetitively traverses the sideband modulation component, and during each traverse selectively passes, or segregates, the various frequency components of the sideband.

It is an object of the present invention to improve the means for segregating the components of a complex wave by providing an improved variable frequency modulating wave.

It is also an object of this invention to improve the oscillatory characteristics of relaxation type oscillators.

In an embodiment of the invention that is to be described in detail herein, there is produced a-variable frequency complex, or square Wave of exceptionally high frequency, and possessing unusual stability With linearity of variation as the control parameters are varied. This wave is obtained from a unique and novel relaxation oscillator of the multivibrator type, the frequency of which may be varied between predetermined maximum and minimum values in substantially equal discrete steps. Control of the changes in the oscillators frequency resides in a stair-step voltage wave generator which utilizes the socalled bucket and dipper method of charging a storage, or sweep, capacitor, and in which the capacitor is discharged in an exceptionally short interval, thereby realizing an unusually short restoration, or fly-back: time. A second control arrangement is provided for the oscillator by means of which the oscillating frequency, at one of its limiting conditions, is checked after each frequency sweep, and corrective adjustments are applied when the measured frequency diifers from a predetermined value.

The nature of the invention and its various features, objects and advantages will be more fully understood from a consideration of the following description of one embodiment, illustrated in the accompanying drawing in which:

Fig. 1 illustrates diagrammatically an arrangement wherein the oscillatory system of the invention may be incorporated as a part of a wave analyzing system;

Figs. 2, 3 and 4 are schematic diagrams of the stair-step wave generator, the relaxation oscillator and the frequency stabilizing circuits, respectively, in accordance with the invention, and incorporated in the arrangement of Fig. 1; and

Fig. 5 indicates the manner in which Figs. 2, 3 and 4 should be arranged to show the complete schematic diagram of the described embodiment.

General description Referring now to Fig. 1, there is shown diagrammatically a portion of a wave analyzing system incorporating the invention. The arrangement of Fig. 1 comprises a source It of signal waves, a modulator l2, an oscillator it, a source it of trains of impulses or pulses of differing durations and differing recurrence rates, a socalled stair-step or staircase-type wave generator l8, a frequency stabilizing circuit 29 including a frequency-checking branch and a frequencycontrolling branch, and a band-pass filter 22. Such an arrangement might, with suitable interconnecting arrangements, be incorporated in the visible-speech portraying arrangement disclosed in the above-mentioned Lacy patent.

More particularly, complex signal waves from source is are supplied to a balanced modulator l2, which may be any suitable known type, along vith square wave oscillations from oscillator I i, the frequency of which is controlled by genera tor it. Two trains of voltage pulses are supplied by pulse source iii. A first train of these, composed of pulses of relatively short duration having any suitable recurrence rate, for example, 490 a econd, is supplied to the generator I8 to cause it to produce an output wave which successively steps, 01' increases in substantially equal valued increments, between predetermined minimum and maximum values as such pulses are received. The pulses of the second train of pulses are of relatively long duration, and have a recurrence rate which varies as the reciprocal of the number of step increments that are to be included in the output wave of generator is. These relatively long pulses are delivered simultaneously to the generator iii and to the frequency checking branch of the frequency stabilizing circuit 29. The stepping operation of generator is is suspended durin the interval of each long pulse of this second train of pulses, and the generator is reset, or reconditioned, to start the production of a new stepped wave at the end of the pulse period. Simultaneously, oscillator id is restored to its original operating conditions, and its output frequency is observed by the frequency-checking branch of stabilizing circuit Ell. This frequency observation or determination is made by observing the voltage that is developed in a frequency-sensitive circuit by oscillations from oscillator l4, and any discrepancy from a predetermined and desired frequency is transformed into a biasing adjustment that, acting through the frequency-control branch of the stabilizing circuit 28, tends to restore oscillator it to its optimum unstepped, or minimum, frequency value. Modulator l2 functions in a conventional manner to produce modulation products from the combined waves. These products are applied to the input of a band-pass filter suitabl proportioned in known manner, the pass-band and center frequency of which are suitably correlated with the frequencies of the signal wave from source it and the modulating wave from oscillator i l. The output from filter 22 may be applied in any desired manner, for example, to control the intensity of a cathodera beam as is done in the aforementioned Lacy patent.

Detailed description A specific structure or circuit arrangement for producing a square wave output, the frequency of which is variable in a series of discrete steps between fixed minimum and maximum values, is shown in Figs. 2, 8 and l, when arranged in the manner shown in Fig, 5.

Referring to Fig. there is shown the stair-step wave generator Ill. The circuit of generator it comprises a plurality of multiple-section electron discharge devices, specifically, the double-triode vacuum tubes it, c, is, 9 and the double-diode vacuum tube 85, and associated resistor and capacitors. The control grid electrode 26 of tube 2 is normally biased positively with respect to its cathode 35, and is connected to pulse input terminal 28 at which the short, sharp, negative voltage impulses Ell are received from pulse source it; (Fig. 1). The left half of tube 24 acts as a polarity inversion amplifier. The right half of tube 25 is normally held at anode current cutoff by the negative potential that is derived by control grid 3i through resistors til, 33. Control electrode 3? is also coupled by way of resistor 3%, capacitor iii; and conductor $8 to pulse input terminal 85, at which the relatively long, widely spaced, positive voltage impulses 3| of the second series of pulses from source It (Fig. 1) are received. Resistors i l form a highprecision voltage divider, to which are connected the anodes of tube 2 5 and the control electrode til of tube 5% Thus, the potential of control electrode is controlled by current conduction in either half of tube 2 1. Resistors 55, and potentiometer 58 form a voltage divider circuit to which the control electrode 52 of tube is connected. When the left half of tube Ed is in its saturated anode current condition, the potential on control electrode 52 is more positive than is the potential on control electrode 38, and the potential of cathode ill of tube to is determined solely by the potential of control electrode 52. Resistor 53 is a cathode load resistor across which is generated a voltage depending upon the current conduction in tube The upper end of resistor 58 is connected over conductor 6| to one terminal of the dipper or transfer capacitor iii. The other terminal of capacitor $2 is con ected over conductor 62;, through the left half of double diode tit, that is, anode e"; and cathode iii and over switch to the sweep, bucket, or storage capacitor 64. Capacitor M is shown as a multisection unit or different values, any one or" which may be chosen by var "ing the position of switch es. The values of the sections of capacitor t l are chosen with regard to the size of the voltage steps, or increments, that are desired in the stepped output wave from generator iii. Although an arrangement for so doing is not specifically shown in the drawing, since it does not form a part of this invention, it should be understood that the pulse recurrence rate of the previously described relatively long voltage impulses 3i, of the second train. of pulses obtained from pulse source 1e (Fig. i), is variable, and that it is coordinated with the position of switch iiii such that as the period between these pulses is made greater or smaller the capacity or capacitor 65L is increased or decreased, proportionately. Thus, the proper coordination of the position of switch 65 and the repetition rate or the lon pulses fit, from source iii, results in the production of an output, or sweep, voltage that varies between the same minimum and maximum values independently of the number of steps or incremental changes in it.

The upper or ungrounded terminal of capacitor o l is connected through switch and over conductor 59 to control electrode it and anode it of the double triode i l. The left half of double triodo id forms a cathode follower, with cathode ll connected through cathode load reslstor iii to a source of negative potential. The upper terminal resistor i6 is connected over conductor "ii to terminal is, where it is available for connection to any suitable utilization circuit, such as the horizontal deflection plates of a cathcde-ray tube (not shown), and is also connected over conductor ll to the bias producing combination comprising capacitor so resistor 32, and through resistor 8i to anode of the right half of diode 66. The cathode (it asso ciated with anode as is connected to anode El. and over conductor 53 to capacitor 62. The bias producing means 80, 82 is provided to prevent undesired charging of capacitor 64 during the interval between pulses at terminal 23. I This charging would otherwise occur since the cathode lI is normally at a higher potential than its control electrode iii, and charging current can flow over the circuit comprising cathode II, conductor 5?, right and left halves of double diode til, conductor 59, switch 65 and capacitor 64. The potential on capacitor 66 is reflected on control electrode 79; and, therefore, unless a blocking bias, such as is developed across the combination cs, 82, is provided, this charging action continues until the left half of double triode I4 becomes overloaded.

Pulses SI from terminal 85 are applied over conductor 88 and through resistor 90 to control electrode 92 of the tube 94. Control electrode 92 is normally held negative with respect to its associated cathode 93 by the negative potential that it derives through resistor 97, Sill. The potential of cathode 93 is normally somewhat positive with respect to that on control electrode 92 because of its connection to the voltage divider comprising resistors 88, 99. The right half of tube 94 is normally held at anode current saturation by the potential its control electrode I09 derives from the voltage divider comprising re sisters 562, ltd. Anode BI is connected to electrode N38 through resistor I92. Anode 95 is connected directly to the control electrode I3 of the right half of tube it. Anode resistor I35 is of such value that when the right half of tube 94 is at anode current saturation, control electrode I3 is held sufliciently negative with respect to the potential existing on its associated cathode I5 so as to render the right half of tube M nonconducting.

Referring now to Fig. 3, the relaxation oscillator I l comprises a plurality of electron discharge devices or vacuum tubes, specifically, triode Ht, tetrodes I66, I08, pentodes I35, E37, I5 3,

unidirectionally conducting devices Q22, EM, key I52, and associated resistors and capacitors. Tubes I05, IE3 may be operated with a relatively low anode and screen grid voltage supply, which in one tested embodiment was about 60 volts, as obtained from voltage regulating tube IIG. Anode I! I of triode H0 is connected to a suitable source of positive potential. Control electrode H2 is connected to the movable contact of a potentiometer H3, which forms a portion of the voltage divider circuit comprising resistors H4, H6. The potential at electrode II2 controls the magnitude of the voltage supplied to anodes I I8, I20 and screen electrodes I I9, I2I of oscillator tubes I06, N33. The cathode electrodes of these latter tubes are connected directly to ground, and a suitable unidirectional conducting device I22, I24, such as a germanium varistor, is shunted between the control electrode and the cathode of each tube. Anode load resistors I 25, I25 are of relatively low valuein order to minimize the effects of tube capacity, and in order to obtain desired saturation characteristics in the tubes IDS, 98. In one tested embodiment of the invention, in which type 807 oscillator tubes were employed, and in which the oscillating frequency was linearly varied from about 1.22 to 2.0 megacycles per second, it was found desirable to use high-quality deposited-carbon type resistors in these anode circuits instead of the more conventional wire-wound units, in order to reduce or eliminate the effects of residual inductance. Control grid electrodes I28, I30 are connected to grid return circuits comprising resistor HI and the upper half of potentiometer E33 and resistor I 32 and the lower half of potentiometer I 33, respectively. The anodes H8, iZt are connected directly to controi electrodes E34, are of cathode followers I35, I31, respectively. Cathode I 38 or" tube 535 includes in its circuit load resistor I39, and is also coupled through capacitor Hi! to control electrode its. This cathode is also connected to one terminal of coupling capacitor Hit, the other terminal of which is connected over conductor M2 to control electrode ill of amplifier tube It! (Fig. 4). Returning to 3, cathode I56 of tube l3? includes in its circuit load resistors l4! and H53, and is also coupled through capacitor M3 to control electrode I28 of tube I98. One terminal of output coupling capacitor we is connected to the cathode end of resistor I 15. The other terminal of this capacitor may be connected over conductor I5! to a suitable utilization circuit, such as the balanced modulator ii of Fig. 1. The midpoint of potentiometer I 33 in the control grid circuits of tubes m5, 5% is connected directly to the cathode electrode :52 of cathode follower tube :54. The control electrode 45-6 of this tube is connected through a decoupling resistor to a suitable potential point on the voltage divider comprising resistors I58, Hit, proportioned such that, together with the other voltages that are derived through resistors let, its relaxation oscillator I i will be biased to a suitable minimum frequency operating value. The common junction point of resistors 5553, its is also connected to starting key I 62, to one contact of which negative potential is connected through resistor I63. Also connected to this common junction point is a voltage dividing resistance network comprising potentiometer I54, resistor 95, and the resistors I8! and ltd. The upper terminal of potentiometer I64 is connected over conductor TI to the cathode II of tube I4 (Fig. 1), from which a voltage that is representative of the charge on capacitor 64 is obtained. The lower terminal of resistor 68 is connected over conductor I70 and contact I of key 43! to the cathode end of load resistor I15 in the frequency control branch of the frequency stabilizing circuit 20 (Fig. 4). The function of the resistance network comprising resistors I55, I69, 96, Ie'l, I63, and potentiometer 55 is to provide a suitable unidirectional potential operating point for the relaxation oscillator, to properly proportion on control electrode I56 the stepped voltage as it is derived from conductor 11, and to provide means for injecting over conductor iii) a frequency controlling voltage derived from the frequency stabilizing circuit 29.

The frequency stabilizing circuit 2Q, shown in Fig. 4, comprises a voltage feedback circuit including electron discharge devices, specifically, vacuum tubes M4, I72 and lid, and a delay circuit comprising a pair of single-trip multivibrator circuits including electron discharge de vices, specifically, vacuum tubes I16, H2. The control electrode H l of vacuum tube its is connected over conductor I42 and coupling capacitor his to cathode 2138 of cathode follower i3 (Fig. 3). Vacuum tube HM forms part of a pentode amplifier, the cathode of which is connected to negative potential through resistor I13. Anode IT! or" tube I M is connected to a tuned circuit comprising inductor H9 and capacitor I89, the values of which are chosen such that the combination resonates at a frequency that is slightly less than the minimum operating frequency of oscillator I4. The operation Q, or quality, of this combination is suitably about 40, and thus the voltage appearing on anode H1 is a reasonably good sine wave, notwithstanding that the excitation voltage on control electrode I'I'I is a highly distorted wave of the square type. Anode Ill is coupled through capacitor I82 to control electrode I33, of the left half of tube I12, the operating bias of which is derived over the movable contact of potentiometer I84, and which may be chosen such that cathode I81 of tube I12 is at a suitable positive potential with respect to ground when relaxation oscillator I4 is operating at its minimum frequency value. In one tested embodiment of the invention in which the minimum frequency value of oscillator I4 was 1.22 megacycles per second, this cathode potential was suitably between and volts. Resistor I83 and capacitor Its comprise a filter having a time constant that is relatively long with respect to the oscillations of oscillator I4, for example, 106 microseconds. Therefore, when control electrode iSS is excited by a voltage Wave from oscillator Id, the potential of cathode I81 increases to a voltage that corresponds very nearly to the peak value of the voltage appearing on this control electrode. The right half of tube H2, including control electrode I98, operates as a cathode follower to repeat across cathode load resistor I22 substantially the same potential that exists at cathode I81. The upper terminal of cathode resistor I92 is connected over conductor :63, the contacts of relay I94, when operated, and through resistor I99 to the storage capacitor use and also to the control electrode it)? of cathode follower tube I'M. Resistor i951 and capacitor IQE are chosen of such values that their time constant is of sufiicient length to eliminate any tendency toward hunting about the optimum minimum frequency by oscillator 1 3. The cathode of tube I'M is connected through contact I of key ISI and conductor I'IJ to the lower terminal of resistor I58, in the control electrode circuit of tube I54 (Fig. 3). Key iiii provides, in the manner indicated, a ready means of initially adjusting the control potentials for suitable operation of oscillator I4 and frequency stabilizing circuit 22. The delay circuit comprising double triode I'M is connected as a single-trip multivibrator, in which the control electrode of the right half of tube I16 is connected to anode potential through resistors 202,

Cathodes 2% are connected to the upper terminal of cathode resistor 268. In the absence of any signal on control electrode 2H3, of the left half of tube i'i'a that half of the multivibrator oscillator is non-conducting. Terminal 85 (Fig. l) is connected over conductor 88 and through coupling capacitor 2M to control electrode are. The anode 2H5 of the left half of tube I76 is coupled through capacitor 2I8 to control electrode 285], and is also coupled through coupling capacitor 22b to control electrode 222 of a second delay circuit consisting of the singletrip multivibrator comprising the double triode iiii. This second multivibrator may be similar in all respects to the previously described unit including double triode I'IB, except that the equivalent of resistor 2E2 is not included in its control grid circuit. This resistor is included in the circuit of control electrode 200 in order to sharpen the voltage impulses that control electrode 222 derives from anode 2I6. It is not 8 needed in the circuit of the second multivibrator. Anode 224 of the right half of tube I18 is coupled directly to control electrode 226 of triode 228, the anode circuit of which includes the winding of relay I94.

Operation The manner in which the circuit of Figs. 2, 3 and 4 operates may be best appreciated from the following description if the functional objectives of the various circuit portions are first recalled. The step-wave generator I3 (Fig. 2) utilizes the so-called dipper and bucket method of charging storage capacitor 6 to produce across the capacitor a potential which changes, in any desired number of substantially equal increments, between predetermined minimum and maximum values. This range of changing potential values is produced once during each predetermined interval. the duration of which is coordinated with the period between the pulses SI of the second series of pulses. At the end of each potential sweep, or cycle of increment changes, it is a function of this generator circuit to discharge the storage capacitor to the same minimum potential value, and to perform this discharge function in the shortest possible time.

The relaxation oscillator I l (Fig. 3) operates to change linearly its oscillation frequency between predetermined minimum and maximum values in accordance with the changes of potential across the storage capacitor, or bucket, of the step-wave generator it. Oscillator It varies its frequency from the same minimum value to the same maximum value, regardless of the number of steps or potential increments that occur during the sweep interval. t may also be noted that the instantaneous frequency of oscillator It is a fixed and stable value during the period of any potential value of the output wave produced by generator It. The output of this oscillator is suitable for combination with a second train of complex waves, such as speech signals, in a utilization circuit, such as a balanced modulator i2 (Fig. 1).

It is the function of the frequency stabilization circuit 20 (Fig. 4) to constantly monitor the output of oscillator I l, and to derive during each of its periods of minimum frequency a voltage indication of that frequency. This voltage indication is supplied to the input of the oscillator It in a manner that tends to control the oscillator at the predetermined optimum value during the next succeeding sweep interval. Because the frequency of oscillator I4 is a varying quantity, it is necessary that the frequency stabilizing circuit 28 perform its functions within the comparatively short interval during which oscillator I4 is operated at its minimum value. To this end, a timing circuit is included for interconnecting the frequency checking and frequency control branches of the stabilizing circuit for a relatively short interval during each sweep cycle of the oscillator.

Returning now to Fig. 2, the pulse inverting left half of tube 24 is normally conducting, and its right half is non-conducting because of the high negative potential that is derived by its control electrode 31 through resistors 39, it. When no negative pulse 30 exists at terminal 28, the potential on control electrode 33 is lower than that on electrode 52, and the potential of cathode 51 of tube 50 is determined by the po tential that electrode 52 derives from potentiometer 56. This value fixes the minimum value of the voltage across resistor 58. When a negative voltage impulse 30 is received at terminal 28, it is inverted in the anode circuit of the left half of tube is, and momentarily increases the potential of electrode Lit to a value determined by the voltage divider 42, 44 such that it exceeds the steady biasing potential on electrode 52. Therefore, the potential of cathode varies between a predetermined minimum value, as determined by the potential derived from potentiometer 55, and a predetermined maximum value as determined by the potential derived from the voltage divider 42, 34. When cathode 5'! changes to its maximum potential value, the potential across capacitor 62 is changed by an amount corresponding to the difference between these values, which change causes charge to flow through capacitor 62, through the left side of double diode 66 comprising anode 5? and cathode 65, and into the sweep capacitor 5s, connected between cathode E8 and ground. The amount of this charge, or the voltage division across the two capacitors 62, 66 will be proportional to the ratio of the capacities of one of the units to the combined capacities of the two units; and so long as the voltage excursion of cathode Si is rigidly maintained, the step size, or increment of potential change across capacitor 64 will be a constant value. The potential across capacitor 6 is reproduced, at a slightly more positive value, across cathode resistor 76, and on connecting circuit 71. Thus, when the voltage of capacitor 54 changes, the sweep voltage potential on conductor 17, which is connected to terminal l3 and to the upper terminal of potentiometer iii-i (Fig. 3), is also changed.

At the termination of the negative voltage impulse on terminal it, the potential of the oathode 5? of tube 5t returns to its minimum value. and capacitor 52 discharges over a path comprising resistors 53, 16, conductor Tl, resistors 85, 82, the right half of tube 56, comprising anode 84 and cathode 69, and conductor 63. Because the potential of capacitor M is repeated, at a slightly more positive value at the cathode H of tube M, capacitor 62 discharges in such manner that its right terminal acquires substantially the same potential as exists across capacitor es. The left terminal of capacitor 52 returns to the minimum value potential of cathode 5'! of tube 56. Therefore, when cathode 51 again assumes its maximum potential value, the change of potential across capacitor 62 will be substantially the same as it was at the time of the previous impulse, and substantially the same increment of potential charge will be added to capacitor 6 3. Thus, the increments of change in the potential across capacitor 64 are substan tially equal as the potential across this capacitor varied between the minimum and maximum potentials. Because of conventional cathodefollower action, the potential of cathode H is somewhat in excess of the potential on control electrode iii and that across sweep capacitor E4. If it were not for the biasing effect of the capacitor tt-resistor 62 combination, this increased potential would cause chargincurrent to flow through the right and left paths of tube to further increase the charge of sweep capacitor 64. This action would be regenerative, and sweep capacitor 54 would gradually acquire an unwanted charge. This efiect is prevented by the blocking bias that is developed across capacitor 86 when capacitor 62 is discharged through resistor 82. The charging current which flows through the right path of tube 66 creates a potential drop across this combination in such manner that the anode 84 of tube 66 is suitably negative with respect to the associated cathode. Thus, in the equilibrium state between voltage impulses on terminal 2&, conduction through the double diode 66 is blocked, since the potential of anode Si is suitably negative with respect to the potential of cathode 68, with which it is in series connection. Resistance Si is not essential to the operation of the circuit, but in one tested embodiment it was found to be useful in reducing a small voltage impulse which might otherwise occur at cathode 'II of tube 74! during the recharging period of capacitor 52.

The sequence of operations which has been described is repeated at the time of each negative voltage impulse on terminal 28, until the voltage appearing at the cathode H reaches the predetermined maximum voltage value. Coincident with this condition, the relatively long positive potential reset impulse 3! is received at terminal 85 from pulse source 16 (Fig. 1), which pulse acts both to reset the stair-step wave generator it by rapidly discharging sweep capacitor iii, and to disable the pulse repeating portion of the circuit comprising tube 24. The pulse repeating portion of the circuit is disabled by the coupling of this positive voltage impulse 35 through capacitor 9t and resistor 39 to control electrode 3? or" tube 24. This action increases the potential of control electrode 3? to the point of anode current saturation, and reduces the potential of control electrode 33 and that of cathode 57 of tube 59 to their minimum values, in the previously described manner. Thus, although stepping impulses 30 may be received on control electrode 26 during the interval of pulse 35, the potential of control electrode d8 is lowered to a value where these pulses are ineffective in controlling the potential of cathode 57. The discharge of sweep capacitor 84 is effective through tube 94 and the right half of tube id. When the positive pulse appears at terminal 85, the normally non-conducting left section of tube M is made conductive, thereby cutting off the normally conducting right half which includes anode 95. The potential rise at anode 95 is sufficient to raise the potential of control electrode E3 of tube 14 positive to a point at which this electrode draws current, thereby reducing the plate resistance of the right half of tube 'ld to its minimum value, and holds this tube at this minimum value until after sweep condenser as, which is connected over conductor 59 to anode E2 of tube M, has been discharged to substantially the potential of cathode ?5. It will thus be seen that the action of tube 94 is to cause the amount of grid current which flows in control electrode of tube '14 to be accurately controlled in order that this current may be taken into acount in the design of the dividing network 5!], l9.

At the termination of the positive voltage impulse 3I, the right half of tube 24 is restored to non-conduction. The left half of this tube is restored to its conducting state, and this portion of the circuit is reconditioned for the reception of the negative voltage impulse so from terminal 23. In addition, anode current saturation is re stored in the right half of tube 9 2; and the right half of tube M, including anode i2 and cathode i5, is returned to non-conduction, thus readying the circuit for another incremental potential change or build-up across the sweep capacitor 64.

As the potential on capacitor 64, and at cathode ll of tube i l, changes, or steps from one value to another, the potential of control electrode E55 of tube Hi l in relaxation oscillator is (Fig. 3) is progressively increased. These poten 1 changes are reproduced across the grid ret resistors lei, !S2 and potentiometer I33, thereby controlling the oscillation frequency of oscillator I l. In the conventional manner of relaxation, or inultivibrator oscillators, only one of tubes lilt, its is conductive at any given instant. The voltage at the mid-point of potentiometer 352 is positive to a degree that both of the tubes Iti'i, Hi8 may possibly be conductive at the start of operations. To overcome this, key use is provided, the transfer contact of which is connected to the common point of a capacitorresistor combination W3, 163 through which negative voltage is supplied. When key it?! is operated, the accumulated negative potential on the capacitor IE3 is placed almost directly on control electrode of tube Ifi l, thereby reducing the potential at the mid-point of potentiometer E33. As the potential of electrode 455 gradually recovers from this negative excursion, the change of potential at the mid-point of potentiometer 533 is such as to start tubes Hi8 and Iiill oscillating in the desired manner. If it is assumed that tube Iilli first regains conduction, the voltage drop across its anode load resistor E is coupled directly through the tube i355 and coupling capacitor IlI to control electrode In of tube Hill. This change of potential across capacitor l ll drives control electrode 530 negative in the usual manner. Capacitor iii no '2 charges through resistor I32 toward the potential that exists at the mid-point of potentiometer I33, which potential is a function of the potential of the control electrode of tube ifi l. When capacitor i li acquires sufiicient charge to raise control electrode Hill to a point where current flows in tube I98, the potentials of anode I29 of tube ltd and of control electrode I3E and cathode Hit of tube I3? are changed in the negative irection, which change is coupled through capacitor M3 to the control electrode I28 of tube Iilt, to render this latter tube cut-off or nonconducting. The voltage rise at anode IE8 of tube ldS is reflected in the control grid-cathode circuit of tube I35, and is coupled. through coupling capacitor Ml to the control electrode iii-9 of tube Hi8.

Capacitor E i! now charges from the source of anode supply through the principal charging path comprising the space discharge path of tube I35, capacitor Ml, varistor H4 and the ground return to the negative terminal of the supply source. A second, and inconsequential, charging path is momentarily provided over the path comprising the space discharge path of voltage regulator llil, resistor $25, the control electrodecathode path of tube 43.5, capacitor II, varistor I24 and the ground return to the supply source. The current flow in this second path is additive to the current flow in the primary path, and forms only a relatively small part of the total charging current. It should be noted that neither anode load resistor I25 nor HE is included in the principal charging path for capacitor Hit. The same is true of the charging path for capacitor Hi3. Both of these capacitors are, therefore, charged, either positively or negatively, primarily through the relatively low impedance space discharge path of tube I35, I31 or I54.

In the type of multivibrator oscillator wherein the conventional capacitive coupling is used between the coupled anode and grid electrodes, it is well recognized that the value of the gridreturn resistor must be greater than that of the anode resistor in a fairly well defined ratio. These values, together with the value of the coupling capacitor, are largely determinative of the maximum oscillation frequency, and, hence, the degrees of stability and linearity that may be expected in the oscillator at a specified operating frequency, or frequencies. ince, in the oscillator of this invention the anode resistor is removed from the frequency determinative branch of the circuit, it follows that it may have its optimum value without regard to the grid resistor. Similarly, since a low impedance space discharge path is substituted for the anode resistor in the capacitor charging circuits, much higher oscillating frequencies may be realized with greatly increased stability of operation and linearity or frequency change.

Since the coupling capacitors l ll, let charged from a low impedance source and ac quire their charge in. a shorter time than is the case in the conventional type of multivibrator oscillator, the potential of the control electrode I28 or ltii is raised more positively than is usual. This increased grid potential results in greatly increased screen current flow, and if no precautionary measure were taken it would adversely afiect the linearity of the change oi frequency as the potential at the midpoint of potentiometer E33 is changed, and would also be detrimental to the tube. To obviate this condition, the unidirectional conducting devices i122, I2 1, which may suitably be germanium varistors of the wellknown type, are connected between the control electrode and the cathode of the respective tube. Therefore, when the highly positive impulse is coupled through coupling capacitor l lI, or I43, this impulse is shunted to ground at the cathode rather than being permitted to increase the potential oi the respective control electrode with the attendant undesirable results.

Output impulses from oscillator is are derived across resistor Hit in the cathode circuit of tube ml, and are coupled through capacitor ll'ai! anconductor IEiI to a suitable utilization circuit which, as was previously stated, may be a bellanced modulator I2 (Fig. 1). A second output is taken from the oscillator through the connection of coupling capacitor ME} to cathode E38 oi tube I35. This output is connected over conductor N32 to the frequency-stabilising circuit of Fig. 4.

Proceeding now to the operation of the frequency-stabilizing circuit of Fig. l, the oscillations on conductor I42 cause substantially sine wave frequency variations at the anode Ill of tube M l. The magnitude of these variations dependent upon how much the oscillating fre quency of oscillator it varies from the resonant frequency of the inductor-capacitor combination H9, IBil, the values of which. are so chosen that they resonate at a slightly lower frequency the minimum operating frequency of oscillator i l. Therefore, as the frequency of oscillator it varies from its minimum value, the amplitude of the alternating component appearing at anode I'll of tube Id l also changes. This change is opposite to the frequency change, that is, with frequency increase the amplitude of the alter hating component decreases. As was previously stated, the control grid bias derived from potentiometer Hi l is chosen such that cathode l8? resides at a predetermined suitable voltage, for

example, ten to twenty volts, positive with respect to ground, when its control electrode is excited by oscillations that are derived when oscillator I4 is operated at its desired minimum frequency. As the frequency of oscillator I4 deviates from its optimum values, the magnitude of the alternating components that appear on control electrode I83 of tube Il2 also deviates, and the potential at the cathode ends of resistors I88 and I92 rises above or falls below the optimum value in reverse relation with variations in the frequency of oscillator I4. During intervals when relay W4 is operated, the potential across cathode resistor I92 is applied through resistor I99 to storage capacitor I96 in the control grid circuit of tube H4, and is isolated on this capacitor when relay I94 is released. The potential across storage capacitor I96 is repeatedacross cathode load resistor H5, and is connected over contact I of key I8I and conductor I IE! to the lower terminal of resistor I68 in the voltage com bining resistance network in the control grid circuit of tube I5 i of oscillator I4. The changes in this fed back voltage are such as tend to re turn oscillator I4 to its desired minimum frequency.

Since the frequency of oscillator I 4 is being successively stepped between predetermined minimum and maximiun values by the potential changes at potentiometer I 54, caused by the stair-step output wave from generator I8 (Fig. 2), it is apparent that the frequency indicating voltage across cathode resistor I92 (Fig. 4) is constantly varying. If this changing voltage were applied to storage capacitor I96, the frequency of oscillation of oscillator I4 would be accordingly varied by this feedback action. This condition is avoided by interconnecting cathode resistor I92 and storage capacitor I93 only during the relatively short interval when capacitor 6 2 (Fig. 2) has its minimum potential value, and oscillator I4 (Fig. 3) has its minimum operating value. To this end, the normally open contacts of relay I94 are in series with the connection between resistor Ifi2 and capacitor I96.

It was previously stated that the positive voltage pulse 3! on terminal 86 effectuated the discharge of storage capacitor 64 and incapacitated, during its period, the pulse repeating function of tube 24 in generator I8. In addition to these 7 functions, the impulse 3I indirectly controls the operation of relay I84. Conduction is started in the left half of the single-trip multivibrator comprising the double triode I76 concurrently with pulse 3!. This condition will prevail for a period that is determined by the time constant of resistor 29 and capacitor 2I8, after which conduction will return to the right half of the multivibrator, and a positive voltage impulse will appear at the anode 216 of tube I15. The duration of the cut-01f period of the right side of this multivibrator is chosen such that oscillator It may be returned from its maximum to its minimum frequency value, and all transients may have subsided, before the storage capacitor I55 is connected to cathode load resistor I92. In one tested embodiment, this time was suitably about seven milliseconds when the duration of the positive pulse on terminal 86 was about twenty-two milliseconds. When the potential of anode 2H5 is increased at the .end of its conduction interval, a positive voltage pulse is transmitted through coupling capacitor 220, which pulse forces conduction in the left half of the single-trip multivibrator comprising tube I18.

Conduction in the left half of I78 cuts off the right half of this tube, and the resulting increased potential at anode 22 of tube 278 is sufiicient to cause triode 228 to operate at anode current saturation; and, since the winding of relay m4 is included in its anode circuit, to operate this relay and thereby complete the circuit between cathode resistor I92 and storage capaci tor I96. At the end of the cut-off period of the right half of triode I18, conduction in triode 223 is interrupted, and the contacts of relay 59 are opened. In this manner, then, at a suitable time, for example, seven milliseconds, after the appearance of the positive voltage impulse on terminal 86 the contacts of relay I94 are closed and the potential of storage capacitor I98 is regulated in accordance with the operation frequency of oscillator It in such manner that this oscillator tends to be returned to its optimum minimum value. This readjusted biasing voltage prevails throughout the following frequency sweep, after which the frequency-checking and bias adjusting process is repeated during the next minimum frequency operating interval.

Although this invention has been described as being incorporated in a specific apparatus, in which various of the circuits parameters have been specifically designated by way of illustrative example, it should be appreciated that the invention is not limited to the stated values nor to the specific structure described herein. Various modifications which do not depart from the spirit and scope of the invention will suggest themselves to those skilled in the art to which this invention pertains.

What is claimed is:

1. In a Wave analyzing system in which a first train of complex signal waves may be modulated with a second train of variable frequency complex waves, the combination which comprises a source of voltage impulses producing first train of relatively short impulses having a specifled recurrence rate and a second train of relatively long impulses having a lower pulse recurrence rate, a voltage generator responsive to said impulses and producing in its output circuit a voltage the magnitude of which increases in substantially equal discrete increments corresponding to individual pulses of said first train of pulses, a relaxation oscillator, means interconnecting said generator and said oscillator such that the operating frequency of said oscillator is responsive to each incremental change in the voltage output from said generator, and means connected to said oscillator and responsive to oscillations therefrom for periodically deriving a voltage the magnitude of which is proportional to the frequency of oscillation of said oscillator during period of minimum frequency, storage means intermittently connected to said last-mentioned voltage deriving means for storing said derived voltage between said periodic derivations and for controlling the frequency of said oscillator in accordance with the magnitude of said stored voltage.

2. In combination, a source of periodically recurring voltage impulses, means responsive to a series of impulses from said source for accuinu lating a voltage charge in substantially equal discrete increments respective said impulses, a relaxation oscillator, means interconnecting said accumulating means and said oscillator whereby the oscillation frequency of said oscillator is reoscillation energy from said oscillator for generating a voltage the magnitude of which is indicative of the frequency of said oscillations, means connected to said oscillator for storing said generated voltage and for controlling the oscillation frequency of said oscillator in accordance with the magnitude of said voltage, and means responsive to a second series of pulses from said impulses source for restoring said accumulating means to its original potential condition and for momentarily connecting said storage means to said voltage generating means whereby the frequency of oscillation of said oscillator is controlled toward an optimum value.

3. In Vail. ble frequency oscillatory circuit, means responsive to actuating voltage impulses for repetitively accumulating a voltage charge in substantialy equal discrete i1 crements, a rela-xation oscillator, an amplifier interconnecting said accumulating means and said oscillator whereby the frequency of oscillation of said oscil-- later is varied between predetermined minimum and maximum values in accordance with the magnitude of each discrete potential increment of said accumulated charge, means connected to said oscillator and responsive to the oscillations thereof for deriving a voltage the instantaneous magnitude of which is indicative of the instantaneous oscillating frequency of the oscillator, means for storing said derived voltage and for controlling said oscillator in accordance with the magnitude of said stored voltage, and means operative only during the period of minimum frequency of said oscillator for interconnecting said storage means and said voltage generating means.

l. A variable frequency oscillatory circuit comprising a pair of oscillator tubes and a pair of amplifier tubes, each or" said tubes comprising at least an anode, a cathode and a control electrode, anode-cathode and control electrodecathode circuits therefor, said anode-cathode circuits each including a source of potential having positive and negative terminals, resistive connections between the positive terminal of said source and the anodes of said oscillator tubes and between the negative terminal of said source and the cathodes of said amplifier tubes, a respective unidirectionally conductive circuit between the control electrode and cathode of each of said oscillator tubes, a conductive coupling between the anode of each oscillator tube and the control electrode of a conjugate amplifier tube, capacitive coupling means between the control electrode of each oscillator tube and the cathode of its non-conjugate amplifier tube, a source of positive potential connected to said capacitive couplings, a frequency-sensitive voltage-generating means capacitively connected to said oscillator and productive of a voltage the magnitude of which is indicative of the oscillation frequency of said oscillator, and means for controlling the charging of said capacitive coupling means from said source of positive potential in accordance with the magnitude of said frequency-indicating voltage.

5. The variable frequency oscillatory circuit of claim at in which said means for controlling the charging of said capacitive coupling means comprises a cathode-coupled amplifier, the conductance of which is responsive to variations in the magnitude of said frequency-indicating voltage.

6. A variable frequency oscillator circuit comprising a first and second oscillator tube and a first and second amplifier tube, each of said tubes comprising at least a cathode, an anode and a control electrode, anode-cathode and control electrode-cathode circuits therefor, said anodecathode circuits including a source of potential having positive and negative terminals and resistive connections between the positive terminal and the anodes of said oscillator tubes, and resistive connections between the negative terminal and the cathodes of said amplifier tubes, a unidirectionally conductive device connected to the cathode of each oscillator tube and conductive of electrons between said cathode and its associated control electrode, a conductive coupling individual to and. connected between the anode of each of said first and second oscillator tubes and the control electrode of said first and second amplifier tubes respectively, a capacitive coupling individual to and connected between the control electrode of each of said first and second oscillator tubes and the cathode of said second and first amplifier tubes respectively, a source or" positive potential connected to each of said individual capacitive couplings, a source of unidirectional voltages, and magnitude of which is variable between predetermined minimum and maximum values in substantially equal incremental steps, means coupled to one of said amplifying tubes for deriving a voltage, the magnitude of which varies as the frequency of said oscillator varies, and means responsive to the combination of said stepped voltage and said derived voltage for controlling the rate at which said capacitive coupling means acquire voltage charges from said source of positive potential.

7. The oscillatory circuit of claim 6 in which said frequency-indicating voltage is derived only during the time said stepped voltage has its minimum value.

8. The oscillatory circuit of claim 5 in which said charging rate controlling means comprises a cathode-coupled amplifier, the control electrode potential of which is controlled by the combination of said frequenc -indicating and said stepped voltages.

9. The oscillatory circuit or" claim 6 in which said means for deriving a irequency indicating voltage comprises an amplifier having an anodeoathode circuit and including in the latter and connected to its anode a parallel inductive-capacitive combination, the frequency of resonance of which is less than the minimum frequency of said variable oscillator, a self-biasing peak rectitying detector oapacitively connected to the anode end or said parallel combination, for deriving a unidirectional voltage in accordance with the magnitude or" the oscillations in said parallel combination, and means for storing and reproducing said derived unidirectional voltage.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,465,355 Cook Mar. 29, 1949 2,500,356 Homrighous Mar. 1%, 1950 2,518,499 Smith Aug. 15, 1950 2,562,694 Brown July 31, 1951 2,577,074 Dickinson Dec. 4, 1951 2,588,083 Doba Dec. 25, 1951 

